Wireline and logging while drilling (LWD) tools measure physical properties of the formations through which a borehole traverses. Such logging techniques include, for example, natural gamma ray, spectral density, neutron density, inductive and galvanic resistivity, acoustic velocity, acoustic calliper, downhole pressure, and the like. Formations having recoverable hydrocarbons typically include certain well-known physical properties, for example, resistivity, porosity (density), and acoustic velocity values in a certain range. In some logging applications it is desirable to determine the azimuthal variation of particular formation properties (i.e., the extent to which such properties vary about the circumference of the borehole). Such information may be utilized, for example, to locate faults and dips that may occur in the various layers that make up the strata. Tools capable of producing azimuthally sensitive information on formation properties are typically identified as imaging tools.
Downhole imaging tools have been available in wireline form for some time. Such wireline tools typically create images by sending large quantities of circumferentially sensitive logging data uphole via a high-speed data link (e.g., a cable). Further, such wireline tools are typically stabilized and centralized in the borehole and include multiple (often times one hundred or more) sensors (e.g., resistivity sensors) extending outward from the tool into contact (or near contact) with the borehole wall. It will be appreciated by those of ordinary skill in the art that such wireline arrangements are not suitable for typical LWD applications. In particular, communication bandwidth with the surface would typically be insufficient during LWD operations (e.g., via known telemetry techniques) to carry large amounts of image-related data. Further, LWD tools are generally not centralized or stabilized during operation and thus require more rugged sensor arrangements.
Several attempts have been made to develop LWD tools and methods that may be used to provide images of various circumferentially sensitive sensor measurements related to borehole and/or formation properties. Many such attempts have made use of the rotation of the BHA (and therefore the LWD sensors) during drilling of the borehole. For example, Holenka et al., in U.S. Pat. No. 5,473,158, discloses a method in which sensor data (e.g., neutron count rate) is grouped by quadrant about the circumference of the borehole. Kurkoski, in U.S. Pat. No. 6,584,837, and Spross, in U.S. Pat. No. 6,619,395, disclose similar methods.
In prior art methods, conventional flux gate magnetometers are utilized to determine the tool face angle of the LWD sensor (which, as described in more detail below, is often referred to in the art as sensor azimuth) at the time a particular measurement or group of measurements are obtained by the sensor. While flux gate magnetometers (also referred to in the art as ring core magnetometers) can be used in borehole surveying applications, such magnetometers have some characteristics that are not ideally suited to imaging applications. For example, flux gate magnetometers typically have a relatively limited bandwidth (e.g., about 5 Hz). Increasing the bandwidth requires increased power to increase the excitation frequency at which magnetic material is saturated and unsaturated. In LWD applications, electrical power is often supplied by batteries, making such power a somewhat scarce resource. For this reason, increasing the bandwidth of flux gate magnetometers beyond about 5 Hz is not practical in many LWD applications. Flux gate magnetometers, therefore, are not well suited for making substantially real-time tool face angle measurements in many LWD settings. There exists a need for sensors and/or sensor arrangements that are suitable for making such real time tool face angle measurements.
Flux gate magnetometers are sensitive instruments requiring careful calibration and handling. Though magnetometers have been used in many LWD and MWD tools, these instruments present design challenges that add to the complexity and expense of the tools. The magnetometers are also relatively expensive, which further compounds this problem. A need exists, therefore, for a more simple, more rugged, and lower cost means for providing substantially real-time azimuthal information in LWD imaging applications.
Moreover, AC and/or DC power is often routed through a drill collar (e.g., from a turbine or a battery pack) to an LWD sensor. The magnetic field about the electrical transmission line is known to interfere with nearby magnetometers. While AC fields may be filtered in certain applications, DC fields are particularly difficult to accommodate. There also exists a need for an arrangement suitable for routing electrical power past magnetic field sensors deployed on a drill collar.